1. Field of the Invention
The present invention provides for significantly improved ultraviolet optical filters. This new class of optical filters benefits from improved optical performance, extended physical longevity, high imaging quality of transmitted radiation, transmitted wavelength stability, and minimal autofluorescence.
2. Background
Optical filters and coatings are passive components whose basic function is to define or improve the performance of optical systems. Applications of optical filters and coatings can be diverse as anti-glare computer screens, laser devices such as ophthalmic surgical lasers, sighting devices, etc.
Many applications and instruments exist where optical filters are used to tune the optical behavior of light in the ultraviolet range (typically wavelengths shorter than 340 nm). Some applications include water purification, blood chemistry analysis, and the chemical evaluation of foods, pollutants, gases, etc. As the wavelength of light becomes shorter in the ultraviolet range, prior art optical filters suffer from numerous disadvantages such as: poor optical performance, limited physical longevity, high autofluorescence, very poor imaging quality of transmitted radiation, and transmitted wavelength instability.
Existing state-of-the-art optical coatings employed for ultraviolet applications have been deposited by Physical Vapor Deposition (PVD) technologies which can be classified generally into two general categories: laminated dielectric films and Metal-Dielectric-Metal (MDM) coatings.
Dielectric film technologies include deposition of soft, marginally adherent multilayer thin films onto various glasses. The films are soft and lack physical durability; most films are also water soluble. These films may consist of materials such as lead fluoride, cryolite (AlF.sub.6 Na.sub.3), zinc sulfide, etc. Coatings also may contain refractory metal oxides which are generally more durable. Yet, standard oxide coatings are physically porous, absorb moisture and therefore are optically unstable when exposed to a varying environment.
To protect these sensitive multilayer optical coatings, they are imbedded into a transparent epoxy by lamination onto other glass substrates. Optical filters made by a soft or hard film deposition include multiple coating layers and laminations, requiring burdensome and relatively costly manufacturing processes. Moreover, the epoxy laminate effectively limits the useful temperature range of the product typically to less than about 100.degree. C. Even more disadvantageously, prior art epoxies discolor and degrade over a short time period when exposed to ultraviolet radiation, rapidly degrading filter optical performance. Additionally, epoxy laminates tend to autofluoresce upon exposure to U.V. radiation. This limited such laminates use in sersitive, critical instrumentation and other sensitive application.
Still further, soft film filters can be sensitive to temperature and humidity and therefore have relatively limited operable lifetimes. Additionally, any laminates will oblate the ability to image through a filter of this type, significantly limiting their application. FIG. 9 depicts the performance of a typical soft-film 340 nm bandpass filter. Curve #1 shows the peak performance of a new filter, with a bandwidth or about 10 nm and transmittance of 35%. Curve #2 is the performance of the same filter after exposure to UV light for 10 hours from a 1500 W mercury arc lamp. As can be readily appreciated, after only 10 hours, transmittance drops to about 3%, or a 91.4% reduction in performance.
The second class of PVD process utilized in the in the prior art to manufacture ultraviolet optical filters employ thin films is designated as MDM (Metal-Dielectric-Metal). MDM filters comprise essentially of a single substrate of fused silica or quartz, upon which a multilayer coating consisting of two materials, a dielectric (e.g. cryolite) and a metal (e.g. aluminum) is deposited. FIG. 10 depicts a simplified cross sectional view of a typical MDM filter 10. As depicted, the filter i:, contained by housing 12, for example of anodized aluminum. A substrate 14, typically of fused silica, is coated with an MDM coating 16. MDM films are soft and easily damaged by moisture and oxygen. The final filter therefore consists of a second fused silica substrate mechanically fixed within a ring assembly with a vacuum or an atmosphere of argon separating the two substrates. A second substrate 18 is provided to define a gap 20, which is evacuated or filled with Argon in order to protect the MDM coating 16. A seal 22 is typically provided to hermetically contain the filter components.
In typical applications, the MDM ultraviolet optical filter is generally a bandpass filter, which will pass a short range of wavelengths and eliminate out-of-band wavelengths by reflection. This type of filter is most commonly employed for shorter UV applications (wavelengths shorter than 300 nm). FIG. 11 depicts the typical spectral behavior of a 270 nm MDM bandpass filter. The property of "induced transmission" governs the optical behavior of the coating. In such a filter, the metal film is induced to transmit energy at a particular design wavelength. See P. H. Berning and A. F. Turner, 47 Journal of the Optical Society of America 230 (1957). MDM filters offer the advantage over soft-coating type filters of eliminating laminating epoxies, thus eliminating performance degradation due to solarization (UV discoloration). The optical performance of MDM filters is, however, rather limited. Typically, the peak transmission of 270 nm to 300 nm bandpass filters is at most about 10-25%. The maximum usable temperature of this filter type is relatively low, typically less than 150.degree. C. If the mechanical filter seal of the filter is in any way violated (physically damaged by use or heat), the filter coating will very rapidly degrade due to the absorption of moisture. Further, if any air is allowed to enter the filter cavity, ultraviolet light exposure will create a small amount of reactive ozone, which will oxidize the coating, rendering the filter unusable.